1. Field of the Invention
This invention pertains to the field of heat sink pressure clip which forms a pressure bond between a heat source and a heat sink.
2. Description of Prior Art
There is a rapidly increasing demand for efficient IR semiconductor lasers operating at ambient or thermoelectric cooler temperatures. Military needs include countermeasures and communications whereas commercial applications focus on remote chemical sensing and drug monitoring, leak detection, chemical process control, and laser surgery. In both of these markets continuous wave (CW) or quasi-CW laser operation is essential and current thermal management techniques are the primary impediment to these types of operation.
Thermal management involves removing heat from a device which, in the case of lasers, critically affects the efficiency and maximum operating temperature. As a semiconductor laser is either electrically or optically excited, excess thermal energy from joule heating, optical heating, hot-carrier relaxation, etc., must be efficiently removed from the laser's active region to minimize degradation of the laser's performance at elevated temperatures. Standard techniques to accomplish this involve soldering the laser to a heat sink using one of a variety of soldering alloys. The heat sink is usually a high thermal conductivity material such as diamond or copper.
A typical semiconductor laser structure consists of a few microns of epitaxially grown laser material (epitaxial-side) containing the active region disposed on a lattice matched substrate. The substrate can be conveniently thinned to a minimum of about 50 microns. Two configurations for soldering a laser to a heat sink are epitaxial-side-up and epitaxial-side-down. Since most of the heat is generated in the active portion of the epitaxial layer, the heat removal is most efficient when the epitaxial layer directly contacts the heat sink, i.e., epitaxial-side-down. While this configuration is the best thermally, it is technically more complicated than the epitaxial-side-up technique and methods must be employed to insure that the facets of the laser are not obscured or contaminated by the solder or its residue. Even when voids, granularity and/or other imperfections in the solder joint do not significantly impede the heat flow, the intrinsic thermal resistance of a solder layer can be significant.
All of the soldering techniques employed for electrically-pumped semiconductor lasers may be used to fabricate optically-pumped lasers as well. A further difficulty occurs when the laser is soldered epitaxial-side-down, in that the only access by the pump laser is through the substrate. This requires that the substrate be transparent to the pump laser, which is often impractical due to other constraints related to fabrication and convenience.
Most of the currently-used soldering and mounting techniques require considerable device processing. The semiconductor and heat sink are typically patterned with layers of different metals and the soldering must be done in a highly controlled environment. Some common problems encountered in epitaxial-side-down soldering are degradation of the laser due to stress or high-temperature processing, breaking upon thermal cycling, contamination of the laser facets, and poor yield associated with the critical nature of the alignment between the laser facet and the edge of the heat sink.
Although the above discussion focused on the IR semiconductor laser application, it should be understood, however, that similar considerations apply equally to semiconductor lasers emitting in other wavelength ranges and to many other optical and electronic devices for which thermal management issues are important, including nonlinear difference frequency generation and high-power electronic devices.
In a specific embodiment, the pressure clip disclosed and claimed herein includes a base and a pressure arm that are connected by a keyhole flexible joint. Force is applied to the pressure arm by the pressure screw which passes through the arm. Mounted at the end of the pressure arm is a plunger which exerts force on the device. The epitaxially grown layer of a laser is pressed against the diamond heat sink which is thermally grounded to the copper mounting block.